“Probiotic drinks work…but no one knows how or why,” was the headline in the Daily Mail today. In the face of some suggestion that probiotic products contain too few “friendly"...

“Probiotic drinks work…but no one knows how or why,” was the headline in the Daily Mail today. In the face of some suggestion that probiotic products contain too few “friendly" bacteria to be effective, the Mail reports that the latest research suggests that they have “a clear effect on the body, changing the make-up of bacteria in the gut and the way the body processes fat”. “Taking the right sort could even help weight loss”, The Daily Telegraph said.

The newspaper stories are based on research in mice that found significant effects of probiotics on their metabolism. As with all animal studies, the relevance of the results to humans is unclear and it cannot be concluded from this study alone that humans who drink bacteria, even if "friendly", will lose weight.

Where did the story come from?

The research was carried out by Doctors Francois-Pierre Martin, Jeremy Nicholson and colleagues from Imperial College London, Nestlé Research Centre in Switzerland and Uppsala University in Sweden. The study received funds from Nestlé and INTERMAP. It was published in the peer-reviewed scientific journal: Molecular Systems Biology.

What kind of scientific study was this?

In this laboratory study, the researchers were interested in the effects of probiotics – food supplements that contain live microorganisms – on metabolism in the body. To investigate this, they performed various tests on two groups of nine mice that had been fed different probiotics and a control group of 10 mice that had not. The experimental mice were known as “human baby flora (HBF) mice” because when they were six weeks old, they were dosed with a mixture of microorganisms that created conditions in the gut that are similar to that seen in formula-fed human babies.

The researchers fed each group of mice a diet containing two different probiotics: one containing the live microorganisms L. paracasei and one containing L. rhamnosus. The mice were given the probiotic along with a basic diet every day for two weeks, then the two groups were compared with the control group who had received a similar basic diet but no probiotics. Where possible, samples of urine and faeces were collected before the mice were killed. Samples were also taken from their liver, small intestine, large intestine and blood for testing.

In order to find out about any differences in metabolism between the groups of mice, the researchers used complex statistical modelling techniques that allowed them to compare the groups using a number of different factors at once. These statistical models take into account the large number of chemicals that are formed during metabolism and how the concentrations of the chemicals are related to each other along the gut. They also consider how the concentrations of all the major gut bacteria change with changing levels of other chemicals.

What were the results of the study?

The researchers found the following: the quantity of Bifidobacteria longum and Staphylococcus aureus (types of bacteria) in the gut were reduced in all the mice fed probiotics compared with the control group of mice. In the mice fed L. rhamnosus, quantities of Bifidobacterium breve, Staphylococcus epidermidis and Clostridium perfringens were also decreased, though there was an increase in the quantity of E. coli.

Using the complex statistical model, the researchers found that the mice who had taken the probiotics were metabolically different to the control group of mice. The main differences were in the types of breakdown products found in the liver, plasma, urine and faeces.

What interpretations did the researchers draw from these results?

The researchers conclude that mice fed on probiotics experienced changes to the ecosystem of bacteria within the gut that led to changes in the way the liver processed fats, to a reduction in the level of certain types of fat circulating in the blood (lipoprotein) and to increased breakdown of sugars (glycolysis).

They also say that probiotics changed the way certain compounds (i.e. amino acids) were broken down. Overall their study demonstrates a link between gut flora and host metabolism and illustrates “the fine relationship between a specific gut microbial population” and fat metabolism.

What does the NHS Knowledge Service make of this study?

By applying statistical modelling techniques to the study of the metabolic process, researchers have demonstrated that, in mice, probiotics have a significant effect on metabolism. The application of these statistical methods will be of interest to the scientific community. However, metabolism in mice is very different to that in humans, so the direct application of these findings to man are unclear.

The research shows that probiotics have an effect on the way that the liver processes lipids. The researchers did not actually measure weight gain or loss in the different groups of mice, so any conclusion that probiotics can have an effect on weight is not supported by the data. To suggest that human weight loss was a feature of this study is misleading. For now, one should rely on the more established, proven ways to lose weight. As one of the lead researchers, Prof Nicholson is quoted as saying in the Telegraph, "No amount of probiotics will help anyone if they are on a ‘supersize me’ diet and sit on their butts all day long watching TV."

The mice in the study were of a particular type, i.e. conditions in the gut of the mice resembled that of formula-fed human babies. In this case, the results cannot be generalised to adult mice or even to young mice outside of a laboratory setting, as they are likely to have a very different diet to the one provided here.

The results of this study cannot provide information about how probiotics might affect humans. They open up an avenue for more research, in particular by making use of modelling techniques in a novel way.

Sir Muir Gray adds...

If you want to lose weight, fit an extra 60 minutes walking into your day, rather than relying on bacteria.